Choosing a Liquid Mixer
Liquid mixing involves combining multiple liquids into a single product. The process can be either batch or continuous.
Miscible liquids are the easiest to mix, blending into one another without resistance. However, it’s important to choose the right mixer for your application.
If the shaft of liquid mixer your mixer points straight down into the tank, it creates strong axial flow and vortexing. This can cause air entrainment and reduces the mixing effectiveness.
High Shear
At the microscale (from a couple to a few millimeters) fluid mixing tends to be nearly exclusively shear driven rather than flow-driven. This radically changes the dynamics of the mixing process from one that is dominated by diffusion to a much more shear-driven one. As such, the design of mixers needs to be very different compared to larger scale processes. This means incorporating Y junctions, T junctions, and three-way intersections in the impeller to maximize interfacial area.
In the case of a high shear mixer this enables it to achieve emulsification, disintegrating, material particle size reduction, and homogenization in a single pass. These processes are usually achieved by a system of stators and rotors that are configured to shear the ingredients at a very high rate causing a vortex which draws the materials into the mixer and then expels them radially through openings or pipes in the stator.
In addition to achieving these mixing processes faster, this type of mixer can also save processing time by eliminating the need for a separate cleaning process between batches with differing formulations. This is referred to as clean in place and is accomplished by using a special type of high shear mixer known as an inline high shear mixer. This type of mixer has a perpendicular inlet to receive the raw ingredients and an axial outlet from where they are discharged.
Low Shear
High shear can be damaging to liquids unless they are able to break down and disperse the shear stress. This is where a Silverson Flashmix excels. It uses counter-rotating mixing to force powder into a liquid stream rather than simply pulling it in, creating shear that is very low but not destructive. This is ideal for applications such as emulsification, high-speed dispersing and particle size reduction.
Another application for this type of mixer is to promote chemical and bioorganic reactions in a liquid by dispersing low-density compressible gas bubbles. This helps increase oxygen transfer rate and can reduce the mixing time needed.
This type of mixing is commonly used to mix liquids with varying viscosity levels. In order to achieve an optimal blend, it is recommended that the process starts with the lower-viscosity liquid. This will allow the mixing equipment to operate at a higher capacity and more efficiently as it doesn’t need to be sized for the highest-viscosity liquid.
Finally, this type of mixing is also commonly used to incorporate powders that tend to float or agglomerate. This can be done using a low pumping and high shear impeller such as the radial or pitch impeller on a Silverson Flashmix, which can break up agglomerates quickly to suspend them in the liquid. This prevents agglomerates from building up on the surface of a vessel, which can lead to fisheyes and lumps that are difficult to wash away with conventional agitators.
High Viscosity
Viscosity is a measure of how much resistance a fluid has to movement. It is usually measured in centipoise (cP) and is a very important mixing property when it comes to choosing the right mixer for an application. High viscosity materials resist deformation a lot more than low-viscous ingredients and require significantly higher power levels to keep them in motion and mix them properly.
The Reynolds number, a dimensionless number that describes the level of turbulence in a liquid, is directly proportional to its viscosity. Typically, lower Reynolds numbers require turbine-type mixers to produce high turbulence and good mixing, while higher Reynolds numbers require more powerful and larger multi-shaft mixers with wider mixing blades to achieve sufficient turbulence in the mixing vessel.
As the viscosity of a liquid increases, it becomes more difficult to achieve adequate shear. As a result, shear-thinning or high-viscosity materials are challenging to mix. They tend to clog equipment, overheat and grind to a halt during processing.
ResonantAcoustic Mixing (RAM) is a game-changer for these types of demanding high viscosity mixes. Unlike mechanical axial/blade mixers that have a limited zone of influence in these applications, RAM uses sound energy to overcome these limitations and deliver effective wetting, incorporation and widespread mixing throughout the entire matrix. It can even reduce processing times by orders of magnitude, overcoming mixing challenges that were previously considered impossible.
Low Viscosity
The properties of fluids vary greatly based on their temperature, pressure, and viscosity. Viscosity is the resistance to flow and is measured in centipoise (cP). For example, water has a low viscosity while tar has a very high one. It’s a property that plays a crucial role in the agitation process because it impacts how easily ingredients flow and blend together.
Mixing highly viscous materials requires a strategic approach. The goal is to achieve Packaging Machinery Supplier a uniform mixture without the formation of agglomerates that are difficult to break apart. Often, the best way to accomplish this is by starting with the lower-viscosity liquid and gradually adding in the higher-viscosity fluid. This allows the mixer to operate at a more efficient level since it doesn’t need to be specifically sized for the highest-viscosity component.
Injecting a thinner liquid into a much thicker one also helps to mix them quickly. According to a recent study published in Physical Review Letters, when the thinner liquid is injected into the thicker one, it forms fingers into the thicker liquid that can quickly mix and disperse the two liquids. This is known as a viscous fingering mechanism. In order to make this work, however, the thinner liquid must be injected at the right speed, the researchers said. Mixing equipment that doesn’t take into account this fact may not be able to achieve the desired results.
